Leishmania organisms are intracellular protozoan parasites of macrophages that cause a wide range of clinical diseases in humans and other animals. In some infections, the parasite may lie dormant, and an infected host may be asymptomatic for many years. In other cases, particularly in immunocompromised individuals, the host may develop one of a variety of forms of leishmaniasis. This disease may be subclinical visceral leishmaniasis (VL) or asymptomatic in nature. Patients with subclinical or asymptomatic disease usually have low antibody titers which fall into the gray zone in immunological tests using whole parasites or parasite lysates. Isolation of parasites from these patients is also extremely difficult. Subclinical patients will in some cases progress to acute disease, but often will self-heal. They exhibit mild symptoms of malaise, diarrhea and intermittent hepatomegaly. Asymptomatic patients, in addition to low antibody titers, also display strong, positive delayed hypersensitivity to leishmanial antigens. Alternatively, leishmaniasis may be manifested as a cutaneous disease, which is a severe medical problem but is generally self-limiting, or as a highly destructive mucosal disease. Finally, and most seriously, the disease may be manifested as an acute visceral infection involving the spleen, liver, and lymph nodes, which is generally a fatal disease. Symptoms of acute visceral leishmaniasis include hepatosplenomegaly, fever, leukopenia, anemia and hypergammaglobulinemia.
Leishmaniasis is a serious problem in much of the world, including Brazil, China, East Africa, India and areas of the Middle East. The disease is also endemic in the Mediterranean region, including southern France, Italy, Greece, Spain, Portugal and North Africa. The number of cases of leishmaniasis has increased dramatically in the last 20 years, and millions of cases of this disease now exist worldwide. About 2 million new cases are diagnosed each year, 25% of which are visceral leishmaniasis (VL).
There are 20 species of Leishmania that infect humans. Of these species, VL is generally caused by L. donovani in Africa, China, the Middle East and India, L. infantium in southern Europe and North Africa, or L. chagasi in Latin America. In general, Leishmania species are transmitted to humans and other mammals, primarily the dog, by the bite of a phlebotomine sand fly.
Early diagnosis of leishmaniasis is crucial for successful treatment, but is difficult to achieve with existing techniques. There are no distinctive signs or symptoms of the disease. Parasite detection methods have been used, but such methods are not sensitive or practical. Current serological tests (using, for example, ELISA or immunofluorescence techniques) typically use whole or lysed parasites, and are generally insensitive and prone to cross-reaction with a variety of other diseases. Such methods often fail to detect the potentially fatal disease early enough to allow effective treatment, since they rely on the detection of antibodies that are present during the acute phase of the disease.
Accordingly, there is a need in the art for more sensitive and specific methods for detecting Leishmania infection, and for identifying those asymptomatic Leishmania infections that are likely to progress to acute visceral infections. The present invention fulfills these needs and further provides other related advantages.